Reduction of External Exposure for Residents from the Fukushima Nuclear Accident by Weathering and Decontamination
Abstract External exposure for residents resulting from the Fukushima nuclear accident was measured in the Marumori and Kosugo regions for the period from September 2011 to March 2013 after the occurrence of the Fukushima nuclear accident. Marumori is a rural settlement, and Kosugo is a suburban city along a freeway. The initial substantial reduction in personal dose equivalent [Hp(10)] for Marumori residents, which was in accordance with the reduction in the air dose rate, was observed. Both values of the Hp(10) results and the air dose rates dropped and remained low during the heavy snow season. The values returned to previous levels and then followed a relatively faster reduction than the radioactive decay rate of 134Cs and 137Cs after the snow had thawed. These faster reductions are considered to be caused by weathering from snow melting or migration of radionuclides down the soil column. However, neither a drop resulting from an accumulation of snow nor faster reduction was observed in Hp(10) for Kosugo residents, except the reduction from the radiocesium decay, although the same reduction tendency as that in Marumori was observed in the air dose rates. The discrepancy between the air dose rate and Hp(10) for Kosugo residents might be caused by dose contributions from the fixed contamination in houses in the suburban environment. The effects of schoolyard decontamination on the Hp(10) values for schoolchildren in Kosugo have been observed in the readings recorded since October 2012, and dose reduction was evaluated as approximately 10–20 μSv/month on average.
Keywords Air dose rate • Decontamination • External exposure • Fukushima accident
• Reduction • Residents • Weathering
The magnitude 9.0 earthquake and tsunami that occurred on 11 March 2011 in Japan resulted in severe damage to the Fukushima Daiichi nuclear power plant (NPP), and this disaster caused a month-long release of radioactive materials into the atmosphere. Aerial measurements reported by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) survey showed that the major radioactive plumes spread in the northwest direction from the NPP, causing significant radionuclide deposition in that area. Marumori and Kosugo, which are located in Miyagi, a neighboring prefecture to Fukushima, are near the border between Miyagi and Fukushima. Marumori is located 46 km northwest of the NPP at the closest approach (Fig. 19.1). A distribution map of the radioactivity concentration in the soil  published by MEXT (Nuclear Regulation Authority after 1 April 2013) showed the presence of several radioactive plumes in this area, and the 137Cs deposition level ranged from 100 to 300 kBq/m2. After the decay of 131I (with a half-life of 8 days), radiation doses since June can be primarily attributed to Cs nuclides. When an initial substantial reduction in the air dose rate is observed, which is expected to occur as a result of weathering, physical decay, and migration of radionuclides down the soil column [2–4], the dose for residents is expected to reduce in the same pattern. External exposure to radionuclides deposited in the environment was in many cases one of the dominant contributions to the total dose to the public after the accidental release of the radioactive material into the atmosphere. Internal exposure also contributed to the total dose. The main pathways of internal
Fig. 19.1 Locations of Marumori and Kosugo. The territories studied in Marumori were geographically divided into three regions: Kawadaira, Hippo and Koya
exposure to residents were inhalation and ingestion. The environmental radioactivity level (fallout)  measured by the prefecture showed that the fallout (131I, 134Cs, and 137Cs) was basically not detectable or that small amounts of radiocesium were sometimes detected in all prefectures including Fukushima from the end of June. Therefore, on 1 September 2011, when measurements were initiated, there was no longer a high possibility of being exposed to the source through inhalation.
In this study, the values of air dose rates (ambient dose equivalents [H*(10)]) in Marumori and Kosugo regions were analyzed to investigate their temporal variation from Cs nuclides. The personal dose equivalent [Hp(10)] for residents in those regions was evaluated using optically stimulated luminescent (OSL) dosimeters. Then, the initial substantial reduction in personal dose for residents was compared with that in the air dose rate. The effect of school, nursery, and community center yard decontamination on the Hp(10) values for residents was evaluated. For area monitoring of penetrating radiation such as gamma rays emitted from Cs nuclides, the ambient dose equivalent [H*(d)], with d = 10 mm, which is written H*(10), is used as the operational quantity. The personal dose equivalent [Hp(d)] is the dose equivalent in the International Commission on Radiation Units and Measurements (ICRU) tissue at a depth d in a human body below the position where an individual dosimeter is worn. For monitoring effective dose, d = 10 mm is recommended, and Hp(d) is written Hp(10).